Targeted distribution systems whose use is possible in the present state of the art generally make use of a conversion of an analog signal, representing stimuli to be transmitted, into a digital data signal. Said conversion is usually followed by an encoding of said data in order to reduce the volume as far as possible without thereby in any way causing this data to suffer significant deterioration. The encoded data is designed to be transmitted by packets according to standardized transmission protocols such as the Internet IPv4 or IPv6 protocols. It has to be recognized however that, although these packet transmission protocols offer considerable advantages, which is what has made them successful, they are intrinsically a source of deterioration caused by the inevitable loss of certain data packets. Furthermore, whatever the chosen encoding technique, the goal would be a compression that cannot be obtained except by eliminating certain characteristics, deemed to be superfluous, of the stimuli to be encoded. Such compression would inevitably generate losses which could have a negative impact on the perception that a recipient will have of the stimuli transmitted to him in encoded form.
Now an individual who, for example, has subscribed to a service for the supply of audiovisual programs will be contractually entitled to expect a minimum level of quality of the images and sounds that are transmitted to him or her, so that a provider of this supply service would have to ensure that such demands are properly met. This cannot be achieved except by carrying out a relevant evaluation of the quality of the stimuli which will be restored to the recipient after transmission. This evaluation would have to be done in taking account of the specific human features of this recipient's faculties of perception.
Various methods have already been elaborated in the past for the separate evaluation of a quality of a signal carrying a stimulus such as a moving picture or a sound. These methods make use for example of a group of persons each of whom has to be exposed to said stimulus and is then requested to assign a grade representing its quality as this person would have perceived it. However, such methods require major logistics and are therefore costly to implement since they require that persons should be brought together in a controlled environment such as a test laboratory in a number sufficient so that a statistical mean of the grades assigned is effectively representative of a mean perception. Furthermore, such methods cannot be used for real-time measurement of quality as perceived by a subscriber at the reception of a stream of stimuli that he would have commanded, so that these methods are not adapted as such to quality control applications in the above-described systems for the targeted distribution of information.
It has also been noted, in the implementation of such methods, the perception that certain types of deterioration could have an impact, on the perception of a person, that cannot be totally decorrelated from a chronological context in which such a deterioration was made to occur. Among other harmful effects, a loss of one or more data packets could cause a discontinuity taking the form of a break in the fluidity of video sequences or a break in a sound signal. This is because, in the event of interruption or appearance of delays in a stream of data packets received by a reception decoder, a receiver usually provides a restoration in a loop of a same stimulus defined by data contained in a buffer memory included in this receiver, so that this stimulus will then be essentially invariant unto the new data packets enable a new stimulus to be reconstituted.
It has been observed that the impact of such discontinuity on the perception of the quality of a stimulus that a person exposed to this stimulus would have would be made to vary as a result firstly of the duration of this discontinuity and secondly of the number of discontinuities having previously affected this same stimulus and therefore having also been previously observed by this same person.
An article “FRAME DROPPING EFFECTS ON USER QUALITY PERCEPTION” published in a report of a WIAMIS conference in Lisbon in April 2004 thus proposes a method of automatic computation of a measurement quantity specifically intended to represent the impact, as perceived by a person exposed to a video signal, that would be produced by discontinuities in a data stream representing this video signal. This method enables the quantifying of a combination of the effects, firstly, of a unitary deterioration caused by each discontinuity and depending on the duration of this discontinuity and, secondly, a possible accumulation of such discontinuities.
The work by the inventors however has shown that a poor impression felt by a person exposed to an invariance of a stimulus depends not only on the duration of this invariance and possible preliminary invariances but also on the conditions in which the invariance disappears to the benefit of a resumption of a normal running of the exposure to the stimulus. This tends to show that the approach described here above is not entirely satisfactory.